U.S. patent application number 12/841608 was filed with the patent office on 2012-01-26 for ac/dc power conversion methods and apparatus.
Invention is credited to Earl W. McCune.
Application Number | 20120020135 12/841608 |
Document ID | / |
Family ID | 45493499 |
Filed Date | 2012-01-26 |
United States Patent
Application |
20120020135 |
Kind Code |
A1 |
McCune; Earl W. |
January 26, 2012 |
AC/DC Power Conversion Methods and Apparatus
Abstract
An AC/DC converter that converts an AC input voltage Vin to a DC
output voltage comprises an inductor, a capacitor selectively
coupled to the inductor, a plurality of switches, and a controller.
The controller configures the plurality of switches, inductor, and
capacitor to operate as a buck converter during times when
Vin>Vout and to operate as an inverting buck converter during
times when Vin<-Vout. The controller modulates the duty cycles
of the plurality of switches to regulate the DC output voltage Vout
to the desired, constant output level.
Inventors: |
McCune; Earl W.; (Santa
Clara, CA) |
Family ID: |
45493499 |
Appl. No.: |
12/841608 |
Filed: |
July 22, 2010 |
Current U.S.
Class: |
363/127 |
Current CPC
Class: |
H02M 7/2176
20130101 |
Class at
Publication: |
363/127 |
International
Class: |
H02M 7/217 20060101
H02M007/217 |
Claims
1. An AC/DC converter for converting an AC input voltage Vin to a
DC output voltage Vout, comprising: an inductor; first and second
switches that, during positive half cycles of the AC input voltage
when Vin>Vout, alternately configure said inductor between
storing energy and supplying current; third and fourth switches
that, during negative half cycles of the AC input voltage when
Vin<-Vout, alternately configure said inductor between storing
energy and supplying current; and a controller configured to
control said first, second, third and fourth switches.
2. The AC/DC converter of claim 1 wherein said controller includes
a comparison circuit configured to compare the AC input voltage Vin
to the DC output voltage Vout.
3. The AC/DC converter of claim 2 wherein said controller further
includes a switch control circuit that controls the switching of
said first, second, third and fourth switches depending on
comparisons of the AC input voltage Vin to the DC output voltage
Vout.
4. The AC/DC converter of claim 1 where said controller includes a
switch control circuit that controls the switching of said first,
second, third and fourth switches depending on the AC input voltage
Vin compared to the DC output voltage Vout.
5. The AC/DC converter of claim 1 wherein during positive half
cycles of the AC input voltage when Vin>Vout said controller is
configured to switch said first switch on and off at frequency f
and duty cycle D, and is configured to switch said second switch on
and off at the frequency f and duty cycle (1-D).
6. The AC/DC converter of claim 5 wherein during negative half
cycles of the AC input voltage when Vin<-Vout said controller is
configured to switch said third switch on and off at the frequency
f and duty cycle D, and is configured to switch said fourth switch
on and off at the frequency f and duty cycle (1-D).
7. The AC/DC converter of claim 6 wherein said controller includes
a pulse-width modulator configured to modulate D and regulate the
DC output voltage Vout.
8. An AC/DC converter for converting an AC input voltage Vin to a
DC output voltage Vout, comprising: an inductor; a capacitor
selectively coupled to said inductor; a plurality of switches; and
a controller that configures the plurality of switches, inductor
and capacitor to operate as a buck converter during times when
Vin>Vout, and configures the plurality of switches, inductor and
capacitor to operate as an inverting buck converter during times
when Vin<-Vout.
9. The AC/DC converter of claim 8 wherein said plurality of
switches includes first and second switches that serve as switching
transistors of the buck converter during the times when
Vin>Vout.
10. The AC/DC converter of claim 9 wherein said plurality of
switches further includes third and fourth switches that serve as
switching transistors of the inverting buck converter configuration
during the times when Vin<-Vout.
11. The AC/DC converter of claim 10 wherein said controller is
configured to: switch said first and second switches on and off at
a common frequency f and at a duty cycle D and duty cycle (1-D),
respectively, during the times when Vin>Vout; and switch said
third and fourth switches on and off at the common frequency f and
at the duty cycle D and duty cycle (1-D), respectively, during the
times when Vin<-Vout.
12. The AC/DC converter of claim 11 wherein said controller
includes a pulse-width modulator configured to regulate the DC
output voltage by modulating the duty cycle D of the first and
third switches and by modulating the duty cycle (1-D) of the second
and fourth switches.
13. A method of converting an AC input voltage Vin to a DC output
voltage Vout, comprising: converting the AC input voltage during
positive half cycles to the DC output voltage Vout using a buck
converter; and converting the AC input voltage during negative half
cycles to the DC output voltage Vout using an inverting buck
converter.
14. The method of claim 13 wherein converting the AC input voltage
to the DC output voltage during the positive and negative half
cycles are performed by modulating duty cycles of switches of said
buck converter and said inverting buck converter.
15. The method of claim 13 wherein converting positive half cycles
of the AC input voltage to the DC output voltage Vout using the
buck converter comprises: determining times when Vin>Vout; and
for times it is determined that Vin>Vout, converting the AC
input voltage to the DC output voltage Vout.
16. The method of claim 13 wherein converting negative half cycles
of the AC input voltage to the DC output voltage Vout using an
inverting buck converter comprises: determining times when
Vin<-Vout; and for times it is determined that Vin<-Vout,
converting the AC input voltage to the DC output voltage Vout.
17. The AC/DC converter of claim 13 wherein the buck converter and
inverting buck converter comprise a conjoined buck and inverting
buck converter circuit that shares a common inductor.
18. An AC/DC converter for converting an AC input voltage Vin to a
DC output voltage Vout, comprising: means for determining times
when Vin>Vout and times when Vin<-Vout; first converting
means for converting the AC input voltage during positive half
cycles to the DC output voltage Vout during times when Vin>Vout;
and second converting means for converting the AC input voltage
during negative half cycles to the DC output voltage Vout during
times when Vin<-Vout.
19. The AC/DC converter of claim 18, further comprising means for
controlling said first converting means and said second converting
means.
20. The AC/DC converter of claim 19 wherein: said first converting
means includes first switching means; said second converting means
includes second switching means; and said controller is configured
to control said first and second switching means to maintain the DC
output voltage Vout at a constant level.
21. An AC/DC converter for converting an AC input voltage Vin to a
DC output voltage Vout, comprising: an inductor; a capacitor; and a
plurality of switches configured to selectively couple and decouple
said capacitor to and from said inductor to convert the AC input
voltage Vin to the DC output voltage Vout.
22. The AC/DC converter of claim 21 wherein said AC/DC converter is
configured to convert the AC input voltage Vin to the DC output
voltage Vout without using a bridge rectifier.
23. The AC/DC converter of claim 21 wherein said AC/DC converter is
configured to step down the AC input voltage to the DC output
voltage Vout without using a step-down transformer.
24. The AC/DC converter of claim 21 wherein said inductor,
capacitor, and switches of said plurality of switches are
configured as a buck converter during times when Vin>Vout.
25. The AC/DC converter of claim 21 wherein said inductor,
capacitor, and switches of said plurality of switches are
configured as an inverting buck converter during times when
Vin<-Vout.
26. The AC/DC converter of claim 21 wherein one or more switches of
said plurality of switches is/are configured to isolate said
capacitor from said inductor during light load conditions, allowing
said capacitor to serve as a power supply for a load during said
light load conditions.
Description
FIELD OF THE INVENTION
[0001] The present invention relates in general to power conversion
and in particular to methods and apparatus for converting
alternating current (AC) to direct current (DC).
BACKGROUND OF THE INVENTION
[0002] Many household and industrial machines and devices are
powered by a direct current (DC) power source that has been
rectified from alternating current (AC) power provided by the AC
mains. The AC-to-DC rectification is typically accomplished using a
bridge rectifier 104 (or "diode bridge") comprised of four diodes
102-1, 102-2, 102-3 102-4 configured as shown in FIG. 1. The bridge
rectifier 104 converts the positive and negative half cycles of the
AC input voltage Vin to a full-wave-rectified waveform of constant
polarity. (See FIGS. 2A and 2B). To produce the desired steady DC
output voltage Vout across a load 108, the rectified waveform is
filtered by a smoothing circuit, which in its simplest form
comprises a smoothing capacitor 106 coupled to the output of the
bridge rectifier 104. The smoothing capacitor 106 functions to
maintain the DC output voltage Vout near the peak voltage Vpeak
during the low portions of the AC input voltage Vin, as shown in
FIG. 2C. Some amount of AC ripple is superimposed on the DC output
Vout, even following filtering by the smoothing capacitor 106. The
ripple may or may not be tolerable, depending on the application.
In applications where it is not tolerable, additional filtering can
be employed to reduce it to an acceptable level.
[0003] The AC/DC converter 100 in FIG. 1 generates a DC output
voltage Vout near the peak voltage Vpeak of the AC input voltage
Vin (see FIG. 2C). However, many applications require a much lower
voltage. For example, many machines and devices require a DC
voltage of 12 volts DC or less but the peak voltage Vpeak of the
center-tapped 120 volts RMS (root mean square) residential mains is
near 170 V. To lower the DC voltage to the required level, a
step-down transformer or DC-DC converter 302 (i.e., "buck
converter") is used. FIG. 3 illustrates use of a DC-DC converter
302. The DC-DC converter 302 comprises a switch (typically a
metal-oxide-semiconductor field effect transistor (MOSFET)) 304, a
diode (or, alternatively, a second MOSFET) 306, an inductor 308, a
filter capacitor 310, and a pulse-width modulator (PWM) control
312. The PWM control 312 controls the opening and closing of the
switch 304 at a fixed frequency f that is much higher than the 60
Hz line frequency (typically greater than 1 kHz). When the switch
304 is turned on, current flows through it, the inductor 308, and
then into the filter capacitor 310 and the load 108. The increasing
current causes the magnetic field of the inductor 308 to build up
and energy to be stored in the inductor's magnetic field. When the
switch 304 is turned off, the voltage drop across the inductor 308
quickly reverses polarity and the energy stored by the inductor 308
is used as a current source for the load 108. The DC output voltage
Vout is determined by the proportion of time the switch 304 is on
(t.sub.ON) in each period T, where T=1/f More specifically,
Vout=DVin(dc), where D=t.sub.ON/T is known as the "duty cycle" and
Vin(dc) is the source DC input voltage provided at the output of
the bridge rectifier 104. The PWM control 312 is configured in a
feed back path, allowing it to regulate the DC output voltage Vout
by modulating the duty cycle D.
[0004] Although the AC/DC converter 300 in FIG. 3 addresses the
inability of the AC/DC converter 100 in FIG. 1 to step down the DC
voltage to a lower DC voltage, it does not address another
well-known problem of conventional AC/DC converters--low power
factor. The power factor of an AC/DC converter is a dimensionless
number between 0 and 1 indicating how effectively real power from
an AC power source is transferred to a load. An AC/DC converter
with a low power factor draws more current from the mains than one
having a high power factor for the same amount of useful power
transferred. A low power factor can result due to the input voltage
Vin being out of phase with the input current Iin or by action of a
nonlinear load distorting the shape of the input current Iin. The
latter situation arises in non-power-factor-corrected AC/DC
converters, such as those described in FIGS. 1 and 3, which as
described above use a diode bridge 104. The filter capacitor 106 of
the AC/DC converter 100 in FIG. 1 (and, similarly, the filter
capacitor 310 of the AC/DC converter 300 in FIG. 3) remains charged
near the peak voltage Vpeak for most of the time. This means that
the instantaneous AC line voltage Vin is below the filter capacitor
106 voltage for most of the time. The diodes 102-1, 102-2, 102-3
102-4 of the bridge rectifier 104 therefore conduct only for a
small portion of each AC half-cycle, resulting in the input current
Iin drawn from the mains being a series of narrow pulses, as
illustrated in FIG. 4. Note that although the input current Iin is
in phase with the AC input voltage Vin, it is distorted and,
therefore, rich in harmonics of the line frequency. The harmonics
lower the power factor, resulting in reduced conversion efficiency
and undesirable heating in the AC mains generator and distribution
systems. The harmonics also create noise that can interfere with
the performance of other electronic equipment.
[0005] To reduce harmonics and increase the power factor,
conventional AC/DC converters are often equipped with a power
factor correction (PFC) pre-regulator. The PFC pre-regulator can be
formed in various ways. One approach employs a PFC boost converter
502 coupled between the bridge rectifier 104 and the DC-DC
converter 302, as shown in the power-factor-corrected AC/DC
converter 500 in FIG. 5. The PFC boost converter 502 comprises an
inductor 504, switch 506, diode 508, output capacitor 510 and a PFC
control 512. The PFC control 512 controls the on and off state of
the switch 506. When the switch 506 is switched on, current from
the mains flows through the inductor 504, causing energy to build
up and be stored in the inductor's magnetic field. During this
time, current to the DC-DC converter 302 and load 108 is supplied
by the charge in the capacitor 510. When the switch 506 is turned
off, the voltage across the inductor 504 quickly reverses polarity
to oppose any drop in current, and current flows through the
inductor 504, the diode 508 and to the DC-DC converter 302,
recharging the capacitor 510 as well. With the polarity reversed,
the voltage across the inductor 504 adds to the source input DC
voltage, thereby boosting the input DC voltage. The PFC boost
converter 502 output voltage is dependent on the duty cycle D of
the on-off switch control signal provided by the PFC control
circuit 512. More specifically, the PFC boost converter 502 output
voltage is proportional to 1/(1-D), where D is the duty cycle and
(1-D) is the proportion of the switching cycle T (i.e., commutation
period) that switch 506 is off. In addition to setting the duty
cycle D, the PFC control 512 forces the DC-DC converter 302 and
load 108 to draw current that on average follows the sinusoidal
shape of the AC input voltage Vin, thereby reducing harmonics and
increasing the power factor of the AC/DC converter 500.
[0006] The power-factor-corrected AC/DC converter 500 is suitable
for many applications. However, it has a number of drawbacks.
First, the AC/DC converter is less efficient than desired,
particularly since the AC-to-DC power conversion requires two
stages--the PFC boost converter 502 front end and the DC-DC
converter 302 final stage. Second, the converter 500 has a large
parts count, including parts necessary to implement the two control
circuits (PFC control 512 and PWM control 312), which increases
design complexity and cost, and makes the converter 500 more
susceptible to failure. Third, the PFC boost converter 502
generates very high voltages, which stress the converter's parts
and raise safety concerns.
[0007] It would be desirable, therefore, to have AC/DC conversion
methods and apparatus that are efficient at converting AC to DC,
avoid power factor degradation attributable to using a bridge
rectifier, do not require voltage boosters to counteract power
factor degradation, and do not have a large parts count.
SUMMARY OF THE INVENTION
[0008] Methods and apparatus for converting alternating current
(AC) to direct current (DC) are disclosed. An exemplary AC/DC
converter that converts an AC input voltage Vin, such as may be
provided by the AC mains, to a DC output voltage comprises an
inductor, a capacitor, a plurality of switches, and a controller.
The controller configures the plurality of switches, inductor, and
capacitor to operate as a buck converter during times when
Vin>Vout and to operate as an inverting buck converter during
times when Vin<-Vout. The controller modulates the duty cycles
of the plurality of switches to regulate the DC output voltage Vout
to the desired, constant output level.
[0009] The AC/DC converter of the present invention converts the AC
input voltage Vin to the DC output voltage Vout directly, i.e.,
without the need for a bridge rectifier or transformer to complete
the AC-to-DC conversion. Direct AC to DC conversion avoids power
factor degradation problems attributable to use of bridge
rectifiers, obviates the need for specialized power factor
correction pre-regulator circuitry, and results in a low parts
count and an energy-efficient design.
[0010] Further features and advantages of the invention, including
descriptions of the structure and operation of the above-summarized
and other exemplary embodiments of the invention, will now be
described in detail with respect to accompanying drawings, in which
like reference numbers are used to indicate identical or
functionally similar elements.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a circuit diagram of a conventional alternating
current to direct current (AC/DC) converter;
[0012] FIG. 2A is a signal diagram of the AC input voltage Vin
applied to the AC input of the AC/DC converter in FIG. 1;
[0013] FIG. 2B is a signal diagram of the unfiltered,
full-wave-rectified voltage waveform produced at the output of the
bridge rectifier of the AC/DC converter in FIG. 1;
[0014] FIG. 2C is a signal diagram of the DC output voltage of the
AC/DC converter in FIG. 1 after having been filtered by a smoothing
capacitor;
[0015] FIG. 3 is a circuit diagram of an AC/DC converter equipped
with a step-down buck converter to step down the DC output voltage
to a level lower than possible using just a bridge rectifier and
smoothing capacitor;
[0016] FIG. 4 is a signal diagram illustrating how the bridge
rectifier used by the AC/DC converters in FIGS. 1 and 3 causes
current to be drawn from the AC power source in narrow pulses that
are rich in harmonics;
[0017] FIG. 5 is a circuit diagram of an AC/DC converter having a
step-down buck converter and a power-factor-correcting boost
converter that compensates for power factor degradation caused by
the AC/DC converter's bridge rectifier;
[0018] FIG. 6 is a circuit diagram of an AC/DC converter, according
to an embodiment of the present invention;
[0019] FIG. 7 is a signal diagram of the AC input voltage Vin
supplied to the AC/DC converter in FIG. 6 and its relationship to
the DC output voltage Vout generated by the AC/DC converter and its
inverse -Vout;
[0020] FIG. 8 is a table showing how the switches of the AC/DC
converter in FIG. 6 are switched and driven, depending on the
instantaneous value of the AC input voltage Vin compared to the DC
output voltage Vout generated by the AC/DC converter in FIG. 6 and
its inverse -Vout;
[0021] FIG. 9 is a circuit diagram illustrating how the AC/DC
converter in FIG. 6 reduces to and operates as a buck converter
during times of positive half cycles of the AC input voltage when
Vin>Vout;
[0022] FIG. 10 is a circuit diagram illustrating how the AC/DC
converter in FIG. 6 reduces to and operates as an inverting buck
converter during times of negative half cycles of the AC input
voltage when Vin<-Vout;
[0023] FIG. 11 is a circuit diagram of a comparison circuit that
forms part of the controller of the AC/DC converter in FIG. 6 and
which compare the AC input voltage Vin to the DC output voltage
Vout to determine times whether Vin>Vout and Vin<-Vout;
and
[0024] FIG. 12 is a circuit diagram of a switch control circuit
that forms part of the controller of the AC/DC converter in FIG. 6
and which operates to control the switching of the switches of the
AC/DC converter in FIG. 6.
DETAILED DESCRIPTION
[0025] Referring to FIG. 6, there is shown an alternating current
to direct current (AC/DC) converter 600, according to an embodiment
of the present invention. The AC/DC converter 600 comprises first,
second, third and fourth switches 602, 604, 606 and 608, an
inductor 610, a smoothing capacitor 612, and a controller 614. The
first switch 602 is coupled between one terminal of the AC input
and a first terminal of the inductor 610; the second switch 604 is
coupled between the first terminal of the inductor 610 and the
opposing-polarity terminal of the AC input; the third switch 606 is
coupled between the AC input and the second terminal of the
inductor 610; and the fourth switch 608 is coupled between the
second terminal of the inductor 610 and the positive DC output
terminal. The controller 614 generates switch drive signals for
controlling the switching of the first, second, third and fourth
switches 602, 604, 606 and 608, depending on the instantaneous AC
input voltage Vin compared to the DC output voltage, and
selectively modulates the duty cycles of the first, second, third
and fourth switches 602, 604, 606 and 608 switches so that the DC
output voltage Vout is maintained at the desired level, as is
explained in more detail below.
[0026] The components of the AC/DC converter 600 comprise discrete
devices, one or more integrated circuit (IC) chips, or a
combination of discrete devices and IC chips. In one embodiment,
the controller 614 and first, second, third, and fourth switches
602, 604, 606 and 608 are integrated in a single IC chip
manufactured in accordance with a standard complementary
metal-oxide-semiconductor (CMOS) fabrication process, with the
first, second, third, and fourth switches 602, 604, 606 and 608
comprising metal-oxide-semiconductor field-effect transistors
(MOSFETs). In another embodiment, the first, second, third, and
fourth switches 602, 604, 606 and 608 are formed in a first IC chip
and the controller is formed in a second IC chip. Whereas the
first, second, third, and fourth switches 602, 604, 606 and 608
comprise silicon-based MOSFETs in the exemplary embodiment just
described, other types of switching devices may be used, including
conventional switches, diodes, relays, or other semiconductor-based
or non-semiconductor-based switching devices. For example, in
applications requiring fast switching speeds,
compound-semiconductor-based transistor devices, such as high
electron mobility transistors (HEMTs) or heterojunction bipolar
transistors (HBTs), may be used to implement the first, second,
third, and fourth switches 602, 604, 606 and 608 switches, instead
of silicon-based MOSFETs. For the purpose of this disclosure, the
term "switch" is used in its broadest sense to include all of these
types of switches and any other suitable switching device. The
inductor 610 and capacitor 612 may also be integrated in the one or
more IC chips, or either or both of these devices may be discrete
devices coupled to external pins of the one or more IC chips.
[0027] The AC/DC converter 600 is configured to directly convert an
AC input voltage Vin, such as may be provided by the AC mains, to a
DC output voltage Vout, without the need for a diode bridge or a
step-down transformer. Direct conversion is accomplished by
controlling and modulating the on/off states of the first, second,
third, and fourth switches 602, 604, 606 and 608 using the
controller 614. More specifically, depending on the instantaneous
AC input voltage Vin compared to the DC output voltage Vout, the
switches are turned on (closed), turned off (opened), driven by a
switch drive signal of duty cycle D, or driven by a complementary
switch drive signal of duty cycle (1-D). The switch drive signal
(labeled "D" in FIG. 6) and the complementary switch drive signal
(labeled "1-D" in FIG. 6) are periodic (or semi-periodic) and have
a common, fixed switching frequency f=1/T, where T is the switching
period. As illustrated in the signal diagram in FIG. 7 and shown in
the switching table in FIG. 8, when Vin>Vout, the first switch
602 is driven by the switch drive signal at a duty cycle
t.sub.ON/T=D, the second switch 604 is driven by the complementary
switch drive signal at a duty cycle (T-t.sub.ON)/T=(1-D), the third
switch 606 is turned off, and the fourth switch 608 is turned on.
When Vin<-Vout, the first switch 602 is turned off, the second
switch 604 is turned on, the third switch 606 is driven by the
switch drive signal at a duty cycle D, and the fourth switch is
driven by the complementary switch drive signal at a duty cycle
(1-D). Finally, when Vin is greater than -Vout but less than Vout,
i.e. when |Vin|<Vout, the first, second, third, and fourth
switches 602, 604, 606 and 608 are turned off.
[0028] The DC output voltage of the AC/DC converter 600 is equal to
D|Vin|, where |Vin| is the absolute value of the instantaneous AC
input voltage. According to one embodiment, the controller 614
modulates the duty cycle D, regulating the DC output voltage Vout
so that it is maintained at a constant level. The duty cycle D may
also be managed to improve the power factor of the AC/DC converter
600. Whereas D is modulated to maintain the DC output voltage Vout
at a constant level in the exemplary embodiment described here, in
general Vout, D, and Vin are all variables. Accordingly, Vout need
not necessarily be maintained at a constant level.
[0029] That Vout=D|Vin| is more readily apparent by understanding
that the AC/DC converter 600 comprises an integrated (i.e.,
conjoined) buck converter and an inverting buck converter. During
the positive half cycles of the AC input waveform when Vin>Vout,
the third switch 606 is off, the fourth switch 608 is on, and the
AC/DC converter 600 reduces to and operates as a buck converter
600A, as illustrated in FIG. 9. The first and second switches 602
and 604 serve as the high-side and low-side switches of the buck
converter and are driven by the switch drive signal at duty cycle D
and complementary switch drive signal at a duty cycle (1-D),
respectively. The first and second switches 602 and 604 therefore
alternately configure the inductor 610 between storing energy and
supplying current during positive half cycles of the AC input
voltage when Vin>Vout, and the DC output voltage Vout=DVin.
[0030] During the negative half cycles of the AC input waveform
when Vin<-Vout, the first switch 602 is off, the second switch
604 is on, and the AC/DC converter 600 reduces to and operates as
what may be referred to as an "inverting" buck converter 600B, as
illustrated in FIG. 10. The third and fourth switches 606 and 608
are driven by the switch drive signal D and complementary switch
drive signal (1-D), respectively. The inverting buck converter 600B
inverts the negative input voltage Vin, alternately configuring, by
the switching action of the third and fourth switches 606 and 608,
the inductor 610 between storing energy and supplying current
during the negative half cycles of the AC input voltage when
Vin<-Vout, to produce an output voltage Vout equal to D|Vin|.
Hence, considering both positive and negative half cycles, the
AC/DC converter 600 produces a DC output voltage Vout=D|Vin|.
[0031] The controller 614 of the AC/DC converter 600 includes a
comparison circuit that continually compares the AC input voltage
Vin to the DC output voltage Vout, to determine whether Vin>Vout
or Vin<-Vout. FIG. 11 is a drawing of an exemplary comparison
circuit 1100 that performs this task. The comparison circuit 1100
comprises first and second comparators 1102 and 1104, an inverting
amplifier 1106, a first voltage divider including resistors 1108
and 1110, and a second voltage divider including resistors 1112 and
1114. The first voltage divider scales the AC input voltage down to
a scaled AC input voltage .alpha.Vin so that the voltage is within
the acceptable input voltage range limit of the first comparator
1102. The second voltage divider scales the DC output voltage down
by the same amount to produce a scaled DC output voltage
.alpha.Vout. The first comparator 1102 compares the scaled AC input
voltage .alpha.Vin to the scaled DC output voltage .alpha.Vout,
producing a high output voltage when Vin>Vout and a low output
voltage when Vin<Vout. The inverting amplifier 1106 inverts the
scaled DC output voltage .alpha.Vout to produce a scaled and
inverted DC output voltage -.alpha.Vout. The second comparator 1104
compares the scaled and inverted DC output voltage -.alpha.Vout to
the scaled AC input voltage .alpha.Vin, producing a high output
voltage when Vin<-Vout and a low output voltage when
Vin>-Vout.
[0032] The controller 614 of the AC/DC converter 600 also includes
a switch control circuit 1200, shown in FIG. 12, which controls the
switching of the first, second, third, and fourth switches 602,
604, 606 and 608. The switch control circuit 1200 comprises an
error amplifier 1202, a pulse-width modulator (PWM) 1204, and
switches 1206-1216 having on/off states that control the switching
of the first, second, third and fourth switches 602, 604, 606 and
608. The error amplifier 1202 compares the DC output voltage Vout
to a precise reference voltage Vref that is equal to and defines
the desired DC output voltage Vout and produces an error signals
.epsilon. based on the difference between Vref and Vout. The PWM
1204 generates the aforementioned switch drive signal (labeled "D"
in FIG. 12) and complementary switch drive signal (labeled "1-D" in
FIG. 12) and modulates D based on the error signal c, thereby
providing the switch control circuit 1200 the ability to regulate
the DC output voltage Vout. The switches 1206-1216 are controlled
by the outputs of the first and second comparators 1102 and 1104 of
the comparator circuit 1100 in FIG. 11 and control the switching
states of the first, second, third and fourth switches 602, 604,
606 and 608, in accordance with the switching table in FIG. 8.
[0033] In the exemplary embodiment above, the switch control
circuit 1200 is described as controlling the opening and closing of
the switches 606, 604, 606 and 608, according to the switching
table in FIG. 8. In another embodiment, the controller 614 is
alternatively or further configured to hold switch 608 open during
light load conditions. (What defines the light load condition is
dependent on the application and established and set during
design.) The remaining switches 602, 604 and 606 are configured to
operate according to the switching table in FIG. 8, or are
configured to not switch at all, with no effect on the load 616.
Hence, during light load conditions, the capacitor 612 serves as
the power supply for the load 616.
[0034] While various embodiments of the present invention have been
described, they have been presented by way of example and not
limitation. It will be apparent to persons skilled in the relevant
art that various changes in form and detail may be made to the
exemplary embodiments without departing from the true spirit and
scope of the invention. Accordingly, the scope of the invention
should not be limited by the specifics of the exemplary
embodiments. Rather, the scope of the invention should be
determined by the appended claims, including the full scope of
equivalents to which such claims are entitled.
* * * * *